Aromatic Nitration with Electrophilic N-Nitropyridinium Cations. Transitory Charge-Transfer Complexes as Key Intermediates

Electrophilic aromatic nitration of various arenes (ArH) is shown to be critically dependent on labile charge-transfer complexes derived from N-nitropyridinium cations. The electrophiles XPyNO₂⁺, with X = CN, CO₂CH₃, Cl, H, CH₃, and OCH₃, form a highly graded series of electron acceptors that produce divers [ArH,XPyNO₂ ⁺] complexes, with charge-transfer excitation energies (hv_CT) spanning a range of almost 50 kcal mol^-1. The latter underlie an equally broad spectrum of aromatic substrate selectivities from the different nitrating agents (XPyNO₂ ⁺), but they all yield an isomeric product distribution from toluene that is singularly insensitive to the X substituent. The strong correlation of the nitration rates with the HOMO-LUMO gap in the [ArH,XPyNO₂ ⁺] complex is presented (Scheme III) in the context of a stepwise process in which the charge-transfer activation process is cleanly decoupled from the product-determining step-as earlier defined by Olah's requirement of several discrete intermediates. This charge-transfer formulation thus provides a readily visualized as well as a unifying mechanistic basis for the striking comparison of XPyNO₂ ⁺ with other nitrating agents, including the coordinatively unsaturated nitronium cation (NO₂ ⁺BF₄ ^-), despite their highly differentiated reactivities.